This invention generally relates to protecting metals from corrosive attack and, more specifically, to a metallic article with improved resistance to corrosion, fatigue, corrosion fatigue and stress corrosion cracking.
A variety of techniques are currently employed to mitigate or eliminate the occurrence of corrosion and corrosion damage. This includes the incorporation of additional metal in the design of a component, the redesign of components to incorporate alloys less susceptible to corrosive attack, the environmental isolation of corrosion-susceptible surfaces with paints, coatings or plating, and the modification of the electro-chemical processes responsible for corrosion.
When it is anticipated that a component will be exposed to a corrosive environment, additional metal may be incorporated in the design to account for the loss of material due to corrosion. This technique does nothing to alter or mitigate the rate at which the component corrodes but rather delays the ultimate failure of the component due to corrosion by the addition of material. As such, this technique is not well suited to applications where component weight is a critical design factor.
If, after deployment, a component is found to be particularly susceptible to corrosion, the component may be withdrawn from service and redesigned utilizing a different material that is more resistant to corrosive attack. However, the redesign of a component is often a costly proposition resulting in the duplication of engineering efforts and equipment downtime and is therefore undesirable.
Another, more common technique to prevent or mitigate corrosion is the application of paints, plating or other types of coatings to the corrosion prone surface. The coatings isolate the surface of the component from the corrosive environment and block the flow of electrons between cathodic regions and anodic regions thereby extinguishing the electro-chemical processes responsible for corrosion. For painting or coating, a variety of non-reactive materials may be used. Paint and coating materials may contain solvents and other toxic chemicals creating a health and environmental hazard in the application and removal of the paint or coating.
Plating provides a more durable coating than do paints and other types of coatings. In plating, corrosion resistant metals such as cadmium or chromium have been commonly used to treat corrosion susceptible surfaces. However, cadmium and chromium plating materials present significant health and environmental risks and plating techniques using these materials are being phased out. Further, while the mechanical barrier produced by coating and plating offers significant protection against corrosion, these treatments are susceptible to damage and require periodic maintenance or reapplication. If the coating barrier is penetrated, the underlying metal is exposed to the corrosive environment and corrosion begins to occur. Coatings used on surfaces susceptible to wear, such as the struts on aircraft landing gear, need to be regularly maintained or replaced in order to provide the proper protection to the underlying structure. Such maintenance is time consuming and expensive and may have undesirable health and environmental risks due to the nature of the materials involved.
Cathodic protection systems seek to control the rate of corrosion of a material by altering the corrosion potential of the metal. Cathodic protection of a metal may be obtained by introducing a current in the material that counteracts the normal electro-chemical reactions responsible for corrosion. Several techniques may be used to cathodically protect a metallic article susceptible to corrosive attack including galvanic coatings, impressed currents/solid state coatings, and external current supplied to the surface to be protected. Of these techniques, galvanic coatings or galvanic couples are commonly used to protect a metallic article from corrosive attack by providing a sacrificial material, in the form of a coating or feature, that will preferentially corrode leaving the metallic article protected. Galvanic protection operates by creating a potential difference between two or more areas in electrical contact with one another. The potential difference causes a current to flow between the areas. This current is designed to counteract an existing corrosion current thereby extinguishing the corrosion reaction at the surface to be protected and promoting corrosion at the sacrificial coating or feature. A galvanic couple is obtained by placing two electrochemically dissimilar metals in electrical contact with one another. The metal with the lower corrosion potential, i.e. the metal that is more susceptible to corrosive attack, is comparatively less noble and will preferentially corrode leaving the other metal protected from corrosive attack. The protected metal has a higher corrosion potential relative to the preferentially corroding metal, and is therefore more noble and less susceptible to corrosive attack.
In addition to the deterioration of a metallic surface by corrosion processes, corrosion or exposure to a corrosive environment may also lead to the premature failure of metallic components. Metallic components subject to continued cyclic loading are prone to fatigue failure. The fatigue life of a component may be significantly shortened by exposure to a corrosive environment. This is due to the fact that damage to the surface of a component as a result of corrosion, such as pitting or inter-granular corrosion, acts as a stress riser or stress concentrator and provides an ideal location for the initiation of fatigue cracks. Fatigue in the presence of corrosion is sometimes referred to as corrosion fatigue.
Further, certain metals are also susceptible to stress corrosion or environmentally assisted cracking. Stress corrosion cracking, or SCC, occurs when a susceptible metal is placed in a corrosive environment and subjected to stress, which may be applied, residual, static or cyclic. Beyond a certain threshold value of stress, stress corrosion cracks develop. The onset of stress corrosion cracking may begin suddenly with little or no prior evidence of material loss due to corrosion. Further, once formed, stress corrosion cracks can lead to mechanical fast fracture causing the sudden and catastrophic failure of a metallic component.
To mitigate component failure due to the conjoint effects of stress and corrosion, it is common to introduce compressive residual stresses in the surface of the metallic component to offset applied or residual tensile stresses. A common practice has been to shot peen the surface of the component to introduce a shallow layer of compressive stress. Alternatively, compressive residual stresses may be introduced in the surface of the component by burnishing, deep rolling, laser shock peening, indenting, or controlled impact peening to obtain greater uniformity and depth of the compressive residual stresses introduced in the component as compared to the random nature of shot peening.
While the use of compressive residual stresses is known to mitigate the effects of stress corrosion cracking, compressive residual stresses do not mitigate or prevent the gross corrosion of the metallic surface. To prevent gross corrosion of a metallic article, it is necessary to rely on the anti-corrosion techniques discussed above. Therefore, for parts susceptible to stress related failure and gross corrosion, it may be necessary to utilize a combination of anti-corrosion techniques and compressive residual stresses to mitigate the effects of each failure mechanism.
Accordingly, a need exists for an inexpensive, environmentally safe and easily incorporated method for producing metallic articles with reduced susceptibility to corrosive attack and improved resistance to fatigue, corrosion fatigue and stress corrosion cracking without requiring the use of additional materials or components.